1. Field of the Invention
The present invention relates to a system for measuring energy subtraction images which is capable of selectively describing a target element using X-rays, and, more particularly, to an imaging system for obtaining energy subtraction images which can eliminate the occurrence of artifacts due to background components and which is suitable for use in the measurement of images at high speed.
2. Description of the Related Art
A system for measuring energy subtraction images which is capable of selectively depicting a target element is discussed in the passage from page 713 to page 718 of "Nuclear Instruments and Methods" in Physics Research A (1986). This system employs synchrotron radiation emitted from an X-ray source, and involves selective depiction of a target element which is attained by obtaining roentgenograms of a subject using monoenergetic X-rays containing X-rays having photon energy levels higher and lower than the K absorption edge of the target element and by calculating the subtraction images of these roentgenograms.
The above-described conventional technique requires rotation of a crystal spectroscope in order to switch over the level of the photon energy of X-rays, and has the following drawbacks.
Firstly, rotation of the crystal spectroscope changes the outgoing direction of the monoenergetic X-ray beam obtained by the crystal spectroscope, changing the direction in which the beam travels through a subject. This causes positions in the subject which are imaged using two types of X-ray photon energy to deviate from each other on a detecting surface. This means that calculation of the energy subtraction images is performed between picture elements which do not exactly correspond. This leads to the occurrence of artifacts, and correction of the positions is therefore necessitated. However, only the positions on the focal plane can be corrected with a high degree of accuracy, and a positioning error occurs with respect to the positions of the subject which are not located on the focal plane in proportion to the distance from the focal plane.
Secondly, in order to obtain an energy subtraction image, it is necessary to image the subject twice using X-rays of different energy levels for such a brief period of time that the movement of the subject can be ignored. In a cardiovascular case, energy level must be switched over within a period of 10 milliseconds or less. However, crystal spectroscopes generally employ a large monocrystal as a major component, and it is impossible to repeat rotation of and stop rotation of the crystal spectroscope in as short a period as 10 milliseconds or less.
Thirdly, the intensity and the monochromaticity of the monoenergetic X-rays obtained by a crystal spectroscope have an exclusive relationship. More specifically, when the intensity of the beam rises, the monochromaticity thereof deteriorates whereas when the monochromaticity improves, the beam intensity lowers. In a case where improvement of the beam intensity takes precedence over improvement of the monochromaticity, it is generally necessary to use X-rays with a large photon energy difference. However, it cannot be assumed that the absorption rates at which tissues other than the target element absorb the two types of X-ray beams may be regarded as the same, and the contrast of the background components cannot be completely eliminated by an energy subtraction image operation.
If X-rays having a small photon energy difference are used for measurement despite their low energy resolution, X-rays having an energy level higher than the K absorption edge and X-rays having an energy level lower than the K edge will be mixed with each other, resulting in a reduction in the contrast of the energy subtraction images and a deterioration in the detection limit of a target element. In the case of a spectroscope having excellent monochromaticity, the beam intensity is lowered, and the quantity of roentgenograms obtained in a short period of time is therefore reduced.